Vinyl acetate ethylene (VAE) copolymer and vinyl acetate (VA) homopolymer dispersions containing N-methylolacrylamide (NMA) as a self-crosslinking functional monomer are often applied to nonwoven substrates to provide good dry and wet tensile strength, as well as good water absorptivity. Examples of such substrates include airlaid nonwoven substrates used for wet wipe end-use applications. Wet wipes have an aqueous composition, such as a lotion, impregnated into the substrate to afford a wet texture, and therefore must have good wet tensile strength.
During the NMA crosslinking, however, formaldehyde is produced as an undesirable by-product. In addition, in many cases formaldehyde is also present in the dispersion prior to crosslinking due to the use of sodium formaldehyde sulfoxylate (SFS) as a redox radical initiator in forming the VAE copolymer. Formaldehyde may also be present due to the use of certain preservatives. The presence of formaldehyde in the dispersion, as well as in the substrate after the crosslinking reaction, is, however, undesirable for both the manufacturer of the substrate as well as the end use consumer. Efforts to use VAE or VA resins not containing NMA or other crosslinking monomers, however, have typically resulted in insufficient wet tensile strength. Thus, a need exists for methods and compositions capable of providing acceptable wet and dry tensile strength while minimizing generation of formaldehyde.
U.S. Pat. No. 7,285,504 B2 discloses nonwoven binders with improved wet tensile strength based on vinyl acetate (co)polymer emulsions which are stabilized with polyvinyl alcohol. The improvement is achieved by incorporation of polyacrylic acid.
EP 0 237 643 A2 discloses formaldehyde-free vinyl acetate/ethylene N-acrylamidoglycolic acid copolymers useful as nonwoven binders.
In U.S. Pat. No. 5,143,954 a nonwoven binder with low-formaldehyde is described, employing an N-methylol functional polymer latex and a formaldehyde-scavenging agent.
U.S. Pat. No. 4,449,978 discloses nonwoven products having formaldehyde content of less than 50 ppm in the nonwoven. In the nonwoven binder N-methylol acrylamide is partially substituted by acrylamide. Ammonium chloride is disclosed as a suitable catalyst for inducing crosslinking of the N-methylol units.
U.S. Pat. No. 5,252,332 describes addition of weak acids like boric acid for the improvement of binders based on polyvinyl alcohol (PVOH). This patent describes a PVOH-containing VA or VAE that is used to impregnate a nonwoven, followed by drying. The boric acid is added in the lotion to provide temporary wet strength to the nonwoven when the nonwoven is wetted with the lotion.
Despite the abovementioned advances, there remains a need for simple and cost-effective ways of providing dry and wet tensile strength to nonwovens while minimizing or eliminating generation of formaldehyde.